1. Field of the Invention
The present invention relates to composite laminates of a sheet material, such as wood veneer, and a substrate comprised of multiple layers of polymer resin sheets, and a method of making such composite laminates. More specifically, the present invention relates to multilayer wood veneer composite laminates, the outermost layer of which is a natural wood veneer, and the substrate of which is a plurality of layers of cellulose-containing plastic sheets. The composite laminate according to the present has the look, feel, heft, and sound when struck of natural wood. The method has a very short cycle time, which increases the utilization of capital equipment.
2. Description of the Prior Art
Wood veneers and other composite laminates made from natural materials have been in use for quite some time. For instance, such functional, as well as aesthetically pleasing, articles such as fine furniture, wood veneer wall coverings and parquet floors are made from laminates of wood veneer. However, because wood is a product of nature it is subject to a wide range of natural variations in color, grain size, grain direction, humidity content, and the like. For instance, single layer wood veneers will warp or twist due to the anisotropy of the natural grain of the wood.
In instances where a thicker, more substantive wood veneer is needed, multiple layers of wood veneers can be glued together to form a wood laminate. However, if the grain of each of the veneer layers is oriented in the same direction, the multilayer veneer laminate will warp in the same fashion as a single layer veneer. Also, excessive warpage will also cause the wood layers to delaminate. Therefore, in multilayer laminates of wood veneers, the veneer layers are staged in alternating "with-grain" and "cross-grain" directions in order to minimize warpage of the finished laminate. However, to properly orient the "cross-grain" layers so as to effectively limit warpage necessitates a costly and labor-intensive "stitching" step. Without the "stitching" step, the finished laminate will exhibit unacceptably high warping or twisting.
Another method to increase the thickness of a wood veneer is to laminate a thin veneer to a more substantial substrate of a different material, such as a plastic sheet. However, one of the problems associated with applying a single layer of veneer over a plastic substrate is accounting for the differences in thermal expansion of the diverse materials. The large difference in the coefficient of expansion between natural woods and plastic resins causes delamination of the layers. Delamination is particularly troublesome when the laminate is subjected to large variations in temperature and/or humidity. This makes such laminates commercially unacceptable.
One way others have addressed this issue is by way of a multi-step process that first forms and bond a wood veneer to a thin aluminum sheet in a compression die and then, in a second step, placing the laminate into a plastic injection mold where, for instance, a glass-filled ABS thermoplastic is injected behind the aluminum sheet, where it acts as a substrate. This process is both cumbersome and uneconomical because it requires extra process steps and tooling costs.
Another problem associated with wood veneer/plastic substrate composites is that they are often perceived as low quality materials due to the sound such composites emit when they are impacted. They sound like plastic sheets, rather than a solid wooden board. In high value applications, such as automotive interiors or furniture applications, this sound is commercially unacceptable. In order to be commercially acceptable, a wood veneer/plastic resin composite laminate must be distinguishable from lower perceived-value plastic parts. The present invention solves this problem by incorporating cellulose into the plastic resin substrate of the presently claimed composite laminate. This gives the present laminate the heft and sound when struck of an all-wood laminate, while the outer wood veneer layer provides the requisite wooden surface finish.
To eliminate the "stitching" step, and to ensure a commercially acceptable and economical wood veneer composite, many investigators have attempted to laminate a natural wood veneer with a protective plastic coating so as to exclude moisture from the veneer (and thereby limit warpage). For instance, U.S. Pat. No. 3,475,261, issued Oct. 28, 1969, to H. V. Ettore et al., describes a method and apparatus for coating plywood laminates with a protective plastic film. The plastic film, preferably a polyvinyl fluoride film, is adhered to the plywood substrate using an adhesive containing epoxy, acrylic, an accelerator, and a catalyst.
Another method commonly used in plywood laminates is to bond a more expensive hard wood outer layer to a less expensive soft wood core. In the common practice, the layers are first cold-pressed together with an adhesive interposed between the layers. The adhesive provides sufficient bond strength at cold temperatures to provide an initial bond between the plywood layers. The cold-pressed laminate is then heat treated to fully cure the adhesive to yield a finished product. U.S. Pat. No. 3,658,622, issued Apr. 25, 1972, to F. Horowitz et al, describes such a method. The method of Horowitz et al utilizes a urea-formaldehyde adhesive which has been modified by the addition of a phenol-formaldehyde resin, and acidified.
U.S. Pat. No. 3,730,828, issued May 1, 1973, to K. D. Meiser, describes a flexible, post-formable paper and plastic resin laminate in which the barrier layer is a sheet of wood fiber which has been impregnated with a fully cured, unplasticized melamine-formaldehyde resin. The laminate has a thickness of from 50 to 60 mils. The laminate is fabricated by assembling a bottom layer of kraft paper, a layer of crepe kraft paper impregnated with a conventional phenol-formaldehyde resin, a plurality of sheets of regular kraft paper impregnated with the same phenol-formaldehyde resin, and a top barrier sheet of kraft paper impregnated with the fully cured resin described above. The layers are then placed in a laminating press and compressed under a pressure of 1,000 pounds per square inch.
Another laminate utilizing an outer wood veneer tissue is described in U.S. Pat. No. 5,073,431, issued Dec. 17, 1991, to A. Martinuzzo. The laminate described in this reference is made of a delicate natural outer layer glued to a thermoformable resin substrate. The laminate can be glued to a cloth backing and incorporated into sewn goods such as handbags. The laminate includes an outer layer of a high quality wood or vegetable tissue veneer, such as cork, a middle layer of a hot melt adhesive, and a lower layer of a thermoforming plastic material such as PVC. Because both the PVC substrate and the veneer tissues are very thin (0.05 to 0.3, and 0.5 to 1.2 mm, respectively), thermoforming of the laminate is done at relatively low temperatures (40.degree. to 85.degree. C.).
However, as the above references show, in order to securely bond rigid wood laminates of more substantial cross-sections requires high pressures and high temperatures in order to ensure that the glue between the laminate layers cures properly. This is very costly in terms of energy usage, and necessitates a longer cycle time (on the order of 6-12 minutes, depending upon the configuration and thickness of the laminate) to allow the heat to fully penetrate the laminate cross-section. This long cycle time results in lower productivity, and under-utilization of capital equipment. Since cycle times are relatively long, additional machinery must be purchased in order to achieve a larger production volume. This, of course, significantly increases capital outlays.
Another problem inherent in the longer cycle times of the conventional processes is the pre-curing of the batch glue. Longer cycle times means that each batch of adhesive remains in the pot for a longer period of time. If the adhesive has a short or unpredictable pot-life, it will often pre-cure prior to finishing the complete run. This is not only very wasteful of adhesive, but lengthens production time due to the additional time needed to formulate a new batch of adhesive to finish the run.
Of course, the best way to solve these problems is to decrease cycle time. A significantly lower cycle time would both fully utilize capital equipment and increases productivity because more cycles can be run on the laminate press or compression mold. Additionally, shorter cycle times would lessen the odds of the adhesive pre-curing since an entire batch of adhesive could be utilized well before the end of its pot-life. The present invention includes a method for preparing a veneer composite laminate in which the mold dwell time can be as short as one minute.